Answer:
The DAC is formed by a Switched Capacitor block whose output is switching in nature (Refer AN2041 for details of the theory of operation of a switched capacitor block). In order to have a continuous output from a switched capacitor block, the output has to be passed through a Sample and Hold circuit which is present in the Analog Out Bus. So, to connect the output of a DAC to a PGA, it has to be first connected to the Analog Bus and then to the PGA.